Soft story construction

Soft story construction may be a concern for anyone living in earthquake country. After the Loma Prieta and Northridge temblors, some apartments, condos, and another buildings got pancaked due to lack of structural strength on the first floor. What is it?

What is soft story construction?

This type of building is when the ground floor is largely pane glass windows, garage doors, carports, or other large openings and there’s living space above. Rather than being mostly a wall, it’s largely openings in the wall.

Soft story construction is not limited to homes. It can also relate to office buildings or store fronts on the first floor with large windows or doors rather than solid walls and more floors above it. Think of an automobile showroom as an example.

The danger arises from the lack of sheer wall in case of a quake. The all glass windows on the first floor, or big carports or garages, are simply not as strong as a wall, and may give way in the case of shaking. For that reason, many are bolstered or reinforced. I wrote about it earlier this week in my Move Move2SiliconValley blog, please read more there: Is your home safe in an earthquake?

 

Soft story construction - condominium units

Soft story construction – condominium units

The less obvious housing includes houses with living space over a garage. It’s very common. And it can be retrofitted by strengthening the sides of the garage door opening.

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Post-Tension Slab Foundations

Post-Tension Slab FoundationPost-tension slab foundations are found in newer homes. Here in the Bay Area, a structure’s foundation needs to withstand not only the load of the building, but expansive soils, and the ubiquitous earthquake. Certain foundations are better at handling these conditions, and are seen more frequently here. One of these which is gaining popularity in new construction is the post-tension slab foundation.

What is a Post Tension Foundation?

Post-tensioning is a technique that was developed and first put to regular use in the 1970s, and approved methods have been published by the Post-Tensioning Institute (PTI), a nonprofit organization, since 1976. Sometimes called post tensioning, or simply PT, this is a type of slab foundation with added reinforcement.

In essence, a slab foundation, aka a slab on grade foundation, is a concrete base only a few inches deep, sitting directly on earth. You might see this for a small shed or playhouse, but larger structures are almost always reinforced, usually with rebar, and a fabric water barrier is lain out before the concrete is poured.

A post-tension slab is reinforced with grids of steel cables cased in plastic sheathes instead of rebar. After the concrete has hardened around them, the cables are pulled taut with hydraulic stressing jacks. This pre-stressing of the concrete creates added compressive strength to the foundation.

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